Supplementation of milk formulae with calcium salts is recommended for low birth weight infants. We observed a serious complication of calcium administration in too high a dosage. Three premature infants (birth weight 1,080-1,900 g and gestational age 28-35 weeks) developed an obstruction of the distal ileum during the second week of life, leading to multiple bowel perforations and development of septic peritonitis in two cases. All three patients required surgical removal of a bolus blocking the intestinal lumen. Analysis of the bolus material from one infant indicated that the formation of soaps from calcium and long-chain fatty acids had caused the obstruction. All three infants had been fed a formula for premature infants with the addition of a recently introduced commercial mineral supplement, resulting in a very high calcium concentration of the feed (40.6 mmol/L). Simulation of syringe feeding demonstrated that even higher calcium concentrations could occur in milk portions containing sedimented calcium compounds.